1. Field of the Invention
The present invention relates generally to a sensor circuit. More particularly, the invention concerns a sensor circuit which is advantageously suited for use in combination with a cell-driven type floppy-disc drive.
2. Description of the Prior Art
Heretofore, the floppy-disc drives have found wide applications as inexpensive high-density external storage devices for various data processing systems and equipments.
In the floppy disc system, a floppy disc (also referred to as diskette) of magnetic flexible sheet material housed within a protective envelope of a predetermined configuration is inserted in a processing unit of the floppy disc drive, where data is read out from or written in the floppy disc.
In conjunction with the floppy disc system, there have heretofore been used various sensor circuits such as a disc-in sensor for detecting whether or not a floppy disc is normally inserted within the floppy disc drive, a write protect sensor for detecting whether data previously stored in the inserted floppy disc is to be protected from writing operation, a so-called track zero sensor for determining whether a magnetic head is positioned on the zeroth track of the disc and others.
In general, the sensor is constituted by a so-called photosensor which comprises an infrared-ray emission diode (hereinafter also referred to as LED in abridgement) and a phototransistor disposed in opposition to the LED with a gap defined therebetween, wherein the presence or absence of an object in the gap is detected in dependence on the output signal state of the phototransistor. For preventing the photosensor from erroneous operation under the influence of ambient illumination or the like, the LED is supplied with a current of magnitude in a range of ca. 15 mA to ca. 20 mA. On the other hand, the phototransistor is so connected that a current on the order of 10 mA may flow therethrough in the conducting state although the current depends on the minimum current required for operation of the sensor circuit.
Under the circumstances, the current required for the operation only of the three types of sensor circuits mentioned above will amount to 75 mA to 90 mA. Accordingly, assuming that these sensor circuits operate at a source voltage of 5 V, electric power on the order of 0.5 W is required.
FIG. 1 of the accompanying drawings shows a typical one of the hitherto known sensor circuit. Since the disc-in sensor, the write protect sensor and the zeroth track sensor are substantially of similar circuit configuration, the description will be made on the assumption that the circuit configuration shown in FIG. 1 is for the zeroth track sensor.
In FIG. 1, a reference symbol 1 denotes an infrared-ray emission diode or LED, 2 denotes a phototransistor, 3 denotes a microprocessor (commercially available under the trade name "TT6677S") which serves as a control circuit, 4 denotes a NOT gate serving as a driver circuit for an interface (not shown) to a controller (not shown), and R.sub.1 and R.sub.2 denote working (bias) resistors for the LED 1 and the phototransistor 2, respectively.
The output signal of the phototransistor 2, i.e. the signal S.sub.1 indicative of detection of the zeroth track in the case of the illustrated example, is applied to an input terminal a of the microprocessor 3. In response to the input signal S.sub.1, the microprocessor 3 outputs a signal S.sub.2 from a terminal b, which signal S.sub.2 is applied to the input of the NOT gate 4, whereby an inverted signal S.sub.3 outputted from a terminal c is utilized for driving the interface (not shown).
Next, operation of the sensor circuit will be described with reference to a program stored in the microprocessor 3 and shown in a flow chart of FIG. 2 of the accompanying drawings.
The microprocessor 3 is adapted to perform ON/OFF control of a spindle motor of a floppy disc drive on the basis of a motor control signal, control of a step motor, control of revolution number (check of revolution number) and the like through steps A.
In this connection, although the control which has to be performed at a high speed is carried out by resorting to a so-called interrupt routine, the ordinary control is performed on the basis of the results of polling (for scanning selection).
Accordingly, the processing program executed by the microprocessor 3 includes a loop for the polling (which will hereinafter be referred to as the main loop). It should be noted that the output signal of the zeroth track sensor, i.e. the output signal S.sub.1 of the phototransistor S.sub.1 is processed this main loop.
For example, upon application of the output signal S.sub.1 of the phototransistor 2 at the terminal a of the microprocessor (at step B), this signal S.sub.1 is processed by excecuting the main loop. More specifically, when the signal S.sub.1 is at a low level as decided at a step C, the microprocessor determines whether the step motor is in predetermined phase at a step D. If so, the signal S.sub.2 of a high level is outputted from the terminal b of the microprocessor at a step E. This signal S.sub.2 is inverted through the NOT gate 4 to the signal S.sub.3 which is supplied to the controller (not shown) by way of the interface (also not shown). Obviously, when the signal S.sub.1 is high (i.e. at high level), the signal S.sub.2 is low.
The output signal S.sub.2 is latched internally of the microprocessor 3 to be held till the succeeding polling.
In the prior art sensor circuit of the type described above, a relatively large current of 15 mA to 20 mA flows through the LED 1 during execution of the main loop for performing the polling, bringing about a power consumption which is not negligible.
In these years, endeavors have been made to develop a portable type floppy disc drive adapted to be energized by a cell or battery in accompaniment with tendency for implementation of the data processing apparatus in a small size at light weight. To this end, the power consumption of the floppy disc drive as a whole has to be reduced as low as possible, which in turn means that the power consumption of the sensor circuits should be suppressed to a possible minimum.
For satisfying the requirement mentioned above, it is indispensable to use the sensor components of low power consumption type. In practice, however, great difficulty is encountered in selecting the components for the sensor circuit which meet all the requirements inclusive of those imposed on the manufacturing cost, sensitivity and others.